Field of the Invention
This invention relates to a process for the purification of unrefined sugar solutions. More specifically, it relates to a process for removing turbidity, color, flavor, and odor from impure sugar solutions which may or may not be subjected to further crystallization.
Soft drink manufacturers virtually always require refined sugar for use in their beverages. However, many countries do not have sufficient refining capacity, and in these countries only mill sugar may be available to certain industries. Before using mill sugar for soft drinks it is necessary to remove from it the turbidity, color, flavor, and odor. The sugar syrups (or simple syrups) are generally made at about 60.degree. Brix.
The Cane Sugar Handbook, Meade and Chen, Tenth Edition, John Wiley and Sons, (New York, 1977) page 129, and references therein, each of which references is herein incorporated by reference, discloses in the manufacture of raw sugar (mill sugar), the dark colored raw cane juice, containing gums, waxes, proteins, organic acid, minerals, and particles of vegetable material, is first treated by adding lime to the hot juice. The lime reacts with the organic acids in the juice and forms an insoluble floc with various colloids and with the phosphates in the juice. The floc, containing impurities, is usually allowed to settle to the bottom of the reaction vessel. Alternatively, the floc may be removed by flotation. Polyelectrolytes are usually added to increase the size of the floc particles, and this "secondary" floc is then conventionally allowed to segregate or caused to rise by aeration, employing nozzle injection systems, high speed pumping or agitation. The floc-flotation-clarified cane juice is then evaporated in a multi-effect vacuum evaporator and crystallized in a vacuum pan.
The mixture of sugar crystals and sugar syrup or massecuite, is subsequently centrifuged to remove most of the dark mother liquor, or molasses, from the crystals. Residual molasses remaining on the crystals may then be removed with a water spray during continuing centrifugation. The greater the volume of wash water used to wash the crystals, the purer the resulting mill sugar will be. On the other hand, the more water used, the more the sugar crystals will dissolve, thus reducing the yield of mill sugar. However, no matter how thoroughly the crystals are washed, they may contain impurities occluded within the crystals. These impurities could be reduced by using more lime and phosphoric acid in the flocculation step, but again, at the expense of yield. Thus, even the best raw sugars (mill sugars) may contain various impurities.
Whereas, mill sugar is suitable in the preparation of products such as candy, bakery products, and sweeteners for coffee or tea, said sugar is undesirable in the production of soft drinks because the color, aroma, flavor, and turbidity of the sugar may affect the character of the soft drinks and shorten their shelf life.
Many sugar mills produce "sulfitated" sugars, wherein the cane juice is treated with sulfur dioxide prior to evaporation. These sugars often have a white appearance, which makes them suitable for certain uses, even though the aroma, flavor, and turbidity may not have been significantly reduced by this treatment. Mill sugar made with other special processing steps, such as extra washing, is called "mill white" or "plantation white," and is also suitable for certain uses. Generally, however, neither sulfitated sugar nor plantation white sugars are pure enough for use in soft drinks, in which higher quality refined sugars are necessary.
In manufacturing refined sugar, crystalline raw sugar, containing a number of undesirable non-sugar constituents, is first washed with water to remove any adhering syrup. The syrup that is washed off the crystals is similar in nature to raw cane juice and is treated separately to recover the sugar from it. The washed sugar is dissolved in water, and the resulting syrup is then clarified by floc-flotation after aerating by conventional means. The clarified syrup is then decolorized with activated carbon, bone char or other appropriate decolorizing substances to give a purified "fine liquor." The fine liquor is then crystallized to yield refined sugar. The degree of refinement depends on the number, and the effectiveness, of the flocculation and decolorization steps.
The flocculation steps using conventional techniques may comprise addition of lime and a phosphate ion source, such as phosphoric acid, to the liquor to form a calcium phosphate floc. The floc is conventionally removed by air flotation. A polyelectrolyte may be added in order to form a secondary floc, thereby increasing the size of the floc particles. Decolorization of the clarified liquor is usually accomplished by passing it through columns of bone char before the final crystallization.
The floc clarification process consists of adding to the dissolved sugar small amounts of lime and phosphoric acid, or lime and soluble phosphate salts or aluminum sulfate. At about neutral pH, the lime and phosphate or aluminum sulfate form an insoluble, primary floc wherein, said floc is calcium phosphate or aluminum hydroxide floc which contains insoluble matter, some of the colloids, and much of the color. The floc cannot be conveniently filtered because of its gelatinous nature. It will settle if given enough time, but it does not compact well enough to obtain a satisfactory yield of clarified syrup. Centrifuging in a continuous centrifuge is not satisfactory either, especially at high sugar concentrations (50.degree. to 60.degree. Brix), probably because the turbulence in the centrifuge breaks the floc particles into smaller particles of a density about the same as or less than the syrup, so that a significant amount of floc is left in the syrup after centrifugation.
Another method for removing the primary calcium phosphate or aluminum hydroxide floc from sugar syrups by flotation with air. Generally, a polyelectrolyte is added to form a secondary floc to which gases will more easily adhere, thus making the flotation more efficient.
For flotation to occur, there must be enough bubbles which either adhere to or are captured by the secondary floc and rise to the surface of the syrup where a scum mat forms. The clarified sugar underneath is passed through a polishing filter and is then ready for use. The scum remaining in the tank contains sugar in an amount sufficient such that recovery of the scum is desirable. The recovery consists of mixing the scum with an appropriate amount of water, said amount of water being equal to or less than all the water required for the next batch. The resultant mixture is then aerated, more polyelectrolyte added, and flotation allowed to take place. A scum mat then forms on the surface. The clarified, dilute sugar solution is removed and may be employed in a new batch of higher Brix syrup. The scum containing some sugar may be discarded or recovered by a further recovery step or steps.
Mechanically generated air bubbles are commonly used in commercial floc flotation in sugar syrups, although carbon dioxide or oxygen bubbles are suitable for flotation of chemical flocs.